1. Field of the Invention
The present invention relates to video discs and, more particularly, to a composite video disc structure having symmetry about its central plane for improving resistance to warping and curling in the presence of changing environmental conditions especially heat and humidity.
2. Description of the Prior Art
Video disc structures are described in U.S. Pat. No. 4,225,873 to Winslow. A video disc structure is also described by John Holmes in his patent application entitled "Video Disc Composition and Method for Molding" filed Aug. 28, 1978, Ser. No. 937,221, now abandoned, and assigned to the assignee of the present invention.
Each of these video disc structures is identified as a nonsymmetrical disc. The nonsymmetrical nature of the video disc arises from the fact that a substrate member carries an information bearing layer on one surface thereof. In the Winslow application, Ser. No. 890,407, now U.S. Pat. No. 4,225,873, a glass substrate member is provided. In the environment to be described hereinafter, the glass member does not absorb water nor does it substantially distort in the relatively low temperatures that will be discussed hereinafter.
The John Holmes application Ser. No. 937,221, now abandoned, describes a videodisc member comprising a plastic substrate with an information bearing layer carried on one side of the substrate. A metallized coating covers the information bearing layer. A protective coating is formed on top of the metallized layer. The Holmes prior art structure is referred to as a nonsymmetrical video disc structure because of the substantially greater thickness of the substrate body when compared to the protective overcoat. Additionally, no portion of the structure is isolated between first and second metal coatings. In the Holmes structure, moisture enters simultaneously into both the substrate and the protective coating. During the moisture absorption process, the single metallized layer detrimentally acts as a vapor barrier and prevents moisture absorption equilibrium by isolating the two water vapor absorbing members on each side of the metallized layer. Significantly more water is absorbed into the substrate member than into the overcoat member because of the relative greater thickness of the substrate member. Because of this difference in water vapor content on each side of the metallized coating, the video disc structure tends to warp during the period of absorption into an umbrella shape with the top portion of the umbrella represented by the thick plastic layer. When equilibrium in moisture content is reached on each side of the metallized layer, no warping occurs unless a plastic flow or other similar stress event is caused to happen.
Transmissive recording discs which carry recordings on two surfaces are described in U.S. Pat. Nos. 3,430,966 and 3,518,442 assigned to the assignee of the present application. These differ in many respects with the structure hereinafter described and claimed. Some of the differences between the prior art structure and the improved structure hereinafter described include the use of information tracks formed by members lying in alternate planes. Such information tracks are formed on each major surface of a central core member. A reflective metal coating is formed over each surface carrying the information tracks for providing not only a reflective surface for the impinging reading beam, but also for acting as a barrier about the central core to control the rate of moisture into and out of the core member.